Climatology of Precipitation and Precipitation Extremes in the United States

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Climatology of Precipitation and Precipitation
Extremes in the United States

• George Taylor, Oregon State University
• Greg Johnson, USDA
• COMAP Symposium 02-1, 02-2
• Presented by Matt Kelsch
• October 2001

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A Spatial Climate Modeling System

• PRISM (Parameter-elevation Regressions on
  Independent Slopes Model)
• Developed by Dr. Chris Daly of the Spatial
  Climate Analysis Service, Oregon State University

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Issues of Scale(Spatial and Temporal)

• Over long averaging times (say, the 30 year
  normal maps), only the most important and
  consistent meteorological factors are evident
• Progressively shorter time spans reveal
  ever-increasing nuances of the atmospheric system

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Spatial Considerations

• Means and extremes are not always correlated,
  especially over large spatial domains, or even in
  small regions with significant climate
  variations
• Portland OR MAP 37.39
• Max24 2.62
• Washington DC MAP 39.00
• Max24 7.19

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Orographic Precipitation Enhancement Factors

• Wind Direction (relative to topography)
• Wind Speed
• Atmos. Moisture (precipitable water)
• Elevation Rise
• Slope Angle

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Precipitation Observations

• First order stations
• Cooperative Observers
• Local Networks
• Forest Service
• Psuedo Sites (based on limited observations)
• Snow Courses (liquid equivalent readings)
• most representative in areas where bulk of
  precipitation falls as snow

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438 stations
lt20 to gt200
PRISM Mean Annual Precipitation Hawaiian
Islands, 1961-90
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Mean Annual Precipitation, Hawaii
4-km grid
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Terrain Influences

• Facets allow for windward/leeward effects
• Vertical Extrapolation applies bias for elevation
  changes between gauge locations

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Olympic Mtns Annual Precipitation Facets On
Vertical Extrapolation On
Inches

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Olympic Mtns Annual Precipitation Facets On
Vertical Extrapolation Off
Inches
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Olympic Mtns Annual Precipitation Facets Off
Vertical Extrapolation Off
Inches
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Olympic Mtns Annual Precipitation
No elevation
Inches
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Olympic Mtns Annual Precipitation Facets On
Vertical Extrapolation On
Inches

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Precipitation Trends (in./decade)
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Extreme Precipitation Climatology
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Precipitation Frequency Definition

• 100 - Year Event An event which has a 1
  percent chance of occurrence IN ANY YEAR NOT ONE
  event every 100 years.

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Probable Maximum Precipitation (PMP)

• Theoretically, the greatest depth of
  precipitation for a given duration that is
  physically possible over a given size storm area
  at a particular geographical location at a
  certain time of the year.

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2/24hr
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Fort Collins, CO 27-28 July 1997
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03Z
0530Z
08Z
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PRISM Product Dissemination

• Web Sites
• OSU www.ocs.orst.edu/prism/prism_new.html
• (Raster and polygon coverages of practically
  everything produced to date (Arc, GRASS)
  documentation metadata DEMs)
• NRCS www.ftw.nrcs.usda.gov/prism/prism.html
• (U.S., Regional and State mean annual
  precipitation cartographic products)

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PRISM Product Dissemination

• Compact Discs
• All precipitation layers for all of the U.S.
  3 CDs (East, Central, West) of the lower 48
  states. Includes Arc Explorer viewing software,
  and all documentation.
• Available from the NRCS-NCGC
• 800-672-5559